This invention relates to a semiconductor laser apparatus and a pumping circuit therefor capable of providing a stable light output therefrom.
The light output from a semiconductor laser apparatus usually fluctuates because the threshold current intensity for laser excitation depends on temperature. It has therefore been known to make use of a so-called automatic power control (APC) circuit in order to obtain a stable light output without regard to changes in temperature. Such an APC circuit may comprise a photodetector and serve to detect a portion of the light output from a semiconductor laser apparatus with the photodetector and to automatically adjust the pump current for the semiconductor laser according to the level of output from the photodetector. This method, however, has several disadvantages. Firstly, it is difficult to obtain a stable light output from the moment immediately after the pumping of the semiconductor laser is started because the stabilization of the light output is obtained by a feedback circuit and there are delay elements due to the time constant of the feedback loop. The method may be suited for the CW operation but in the case of pulse pumping, the on-off control of the semiconductor laser may be carried out, for example, by integrating the output pulse signals from the photodetector and detecting changes in the integrated output. Thus, the control circuit inclusive of the photodetector will have a complicated structure as a whole. Secondly, an excessively large current may flow into the semiconductor if there is an abnormality in any of the elements of the feedback loop but there is no adequate means to protect the semiconductor laser therefrom. Thirdly, the circuit structure as a whole becomes complicated. Not only does this adversely affect the cost, but it will also be difficult to miniaturize the apparatus.
In view of the first of the problems described above, Japanese Patent Publications Tokkai 59-17291 and Tokkai 8-316560 disclosed circuits adapted to carry out temperature compensation by means of an open loop and to thereby avoid a time delay associated with the detection of light and the feedback circuit. In view of the second of the problems, Japanese Patent Publication Tokkai 3-145171 disclosed a protective circuit which detects the pump current of the semiconductor laser by means of a current-detecting resistor and switches off the transistor which controls the current to the semiconductor laser when the detected value increases past a specified level. The circuit disclosed in Tokkai 8-316560, however, includes a large number of components such as differential amplifiers, control transistors, resistors of all kinds, diodes and thermistors and hence cannot overcome the third of the problems described above. Neither is the protective circuit disclosed in Tokkai 3-145171 capable of solving the third problem described above because it must be used together with a circuit forming a feedback loop by using a prior art photodetector. As for the circuit disclosed in Tokkai 59-17291, it has no protective means against abnormal heating of the semiconductor laser due to a rise in the environmental temperature or an overcurrent.